U.S. patent number 9,447,520 [Application Number 13/696,611] was granted by the patent office on 2016-09-20 for gas-phase synthesis method for forming semiconductor nanowires.
This patent grant is currently assigned to QUNANO AB. The grantee listed for this patent is Knut Deppert, Magnus Heurlin, Martin Magnusson, Lars Samuelson. Invention is credited to Knut Deppert, Magnus Heurlin, Martin Magnusson, Lars Samuelson.
United States Patent |
9,447,520 |
Samuelson , et al. |
September 20, 2016 |
Gas-phase synthesis method for forming semiconductor nanowires
Abstract
The present invention provides a method and a system for forming
wires (1) that enables a large scale process combined with a high
structural complexity and material quality comparable to wires
formed using substrate-based synthesis. The wires (1) are grown
from catalytic seed particles (2) suspended in a gas within a
reactor. Due to a modular approach wires (1) of different
configuration can be formed in a continuous process. In-situ
analysis to monitor and/or to sort particles and/or wires formed
enables efficient process control.
Inventors: |
Samuelson; Lars (Malmo,
SE), Magnusson; Martin (Malmo, SE),
Deppert; Knut (Lund, SE), Heurlin; Magnus
(Furulund, SE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Samuelson; Lars
Magnusson; Martin
Deppert; Knut
Heurlin; Magnus |
Malmo
Malmo
Lund
Furulund |
N/A
N/A
N/A
N/A |
SE
SE
SE
SE |
|
|
Assignee: |
QUNANO AB (Lund,
SE)
|
Family
ID: |
44914587 |
Appl.
No.: |
13/696,611 |
Filed: |
May 11, 2011 |
PCT
Filed: |
May 11, 2011 |
PCT No.: |
PCT/SE2011/050599 |
371(c)(1),(2),(4) Date: |
January 10, 2013 |
PCT
Pub. No.: |
WO2011/142717 |
PCT
Pub. Date: |
November 17, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20130098288 A1 |
Apr 25, 2013 |
|
Foreign Application Priority Data
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|
|
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May 11, 2010 [SE] |
|
|
1050466 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B
25/005 (20130101); C30B 29/403 (20130101); C30B
25/00 (20130101); B82Y 40/00 (20130101); C30B
29/42 (20130101); B82Y 30/00 (20130101); C30B
29/40 (20130101); C30B 29/62 (20130101) |
Current International
Class: |
C30B
25/02 (20060101); C30B 25/00 (20060101); B82Y
30/00 (20110101); B82Y 40/00 (20110101); C30B
29/40 (20060101); C30B 29/62 (20060101) |
Field of
Search: |
;117/84,86-89,93,102,104,105,937,954 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
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|
|
2004-507104 |
|
Mar 2004 |
|
JP |
|
2007-527844 |
|
Oct 2007 |
|
JP |
|
02076887 |
|
Oct 2002 |
|
WO |
|
2004004927 |
|
Jan 2004 |
|
WO |
|
2004038767 |
|
May 2004 |
|
WO |
|
WO2005/027201 |
|
Mar 2005 |
|
WO |
|
2007102781 |
|
Sep 2007 |
|
WO |
|
Other References
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.
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applicant .
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applicant .
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nanoscale electronic and optoelectronic devices", Nature, vol. 409,
pp. 66-69 (2001). cited by applicant .
Hu, J. et al., "Chemistry and Physics in one Dimension:Synthesis
and Properties of Nanowires and Nanotubes", Accounts of Chemical
Research, ACS, vol. 32(5), pp. 435-445 (1999). cited by applicant
.
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nanophotonics", Small, Wiley-VCH Verlag GmbH & Co. KGaA, vol. 1
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Nanostructures into Functional Networks", Science, vol. 291, pp.
630-633 (2001). cited by applicant .
Lauhon L., et al., "Epitaxial core-shell and core-multishell
nanowire heterostructures", Nature, vol. 420, pp. 57-61 (2002).
cited by applicant .
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Crystalline Semiconductor Nanowires", Science, vol. 279, pp.
208-211 (1998). cited by applicant .
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complexes in the synthesis of shaped carbon nonomaterials", J. of
Organometallic Chemistry, vol. 693, pp. 2205-2222 (2008). cited by
applicant .
Qian, F. et al.,"Gallium Nitride-based nanowire radial
heterostructures for nanophotonics", Nano Letters, vol. 4 (10), pp.
1975-1979. cited by applicant .
Wang, J. et al., "Highly polarized photoluminescence and
photodetection from single indium phosphide nanowires", Science,
vol. 293, pp. 1455-1457 (2001). cited by applicant .
Wu, Y. et al., "Controlled Growth and Structures of Molecular-Sacal
silicon nanowires", Nano Letters vol. 4(3), pp. 433-436 (2004).
cited by applicant .
Extended European Search Report and written opinion received in
connection with European application No. EP 11780888.1, dated Nov.
15, 2013. cited by applicant .
S.H. Kim et al. (Aug. 2007) "Understanding ion-mobility and
transport properties of aerosol nanowires" Journal of Aerosol
Science: vol. 38, Issue 8, pp. 823-842: pp. 823-826; figure 1.
cited by applicant .
C.S. Kim et al, (2010) "Generation of Charged Nanoparticles During
the Synthesis of Silicon Nanowires by Chemical Vapor Deposition" J
. Phys. Chem. C, 114 (8), pp. 3390-3395; pp. 3390-3392; figures
1,3. cited by applicant .
Magnusson et al. (1999) "Gold Nanoparticles: Production, Reshaping,
and Thermal Charging" J Nanoparticle Res 1, 243-251. cited by
applicant .
International Search and Written Opinion issued on Jun. 29, 2011,
in corresponding PCT Application No. PCT/SE2011/050599. cited by
applicant .
Japanese Patent Office Final Rejection for Japanese Patent
Application No. 2013-510045, dated Oct. 6, 2015, 2 pages. cited by
applicant .
Second Chinese Office Action for Chinese Application No.
201180034310.X, mailed on Aug. 24, 2015; with English-Language
Translation. cited by applicant .
Third Chinese Office Action for Chinese Application No.
201180034310.X, mailed on Apr. 21, 2016; with English-Language
Translation. cited by applicant.
|
Primary Examiner: Bratland, Jr.; Kenneth A
Attorney, Agent or Firm: The Marbury Law Group PLLC
Claims
The invention claimed is:
1. A method for forming nanowires comprising: providing metal
catalytic seed particles suspended in a gas, providing Group III
and Group V gaseous precursors, that comprise constituents of the
nanowires to be formed, dissolving the Group III material into the
metal catalytic seed particles at a temperature of 380.degree. C.
to 700.degree. C.; making at least one seed crystal at the surface
of the at least one catalytic seed particle, and growing
epitaxially at least one nanowire crystal from the at least one
formed seed crystal in a gas-phase synthesis including the gaseous
precursors while the catalytic seed particles are suspended in the
gas and the constituents of the nanowires to be formed are
supersaturated in the at least one catalyst seed particle, wherein
the at least one nanowire crystal is a III-V semiconductor crystal
which comprises gallium and arsenic.
2. The method of claim 1, wherein the nanowires are formed in a
continuous process.
3. The method of claim 1, wherein the nanowires formed are carried
by the gas.
4. The method of claim 1, wherein the growth conditions during
growth of each nanowire are varied by controlling one or more of
parameters associated with: precursor composition, precursor molar
flow, carrier gas flow, temperature, pressure or dopants, such that
a nanowire segment is axially grown on a previously formed nanowire
portion in a longitudinal direction thereof, or a shell is radially
grown on the previously formed nanowire portion in a radial
direction thereof, or material is added as a combination of axial
and radial growth.
5. The method of claim 4, wherein the growth conditions are varied
to obtain heterostructures with respect to composition, doping,
conductivity type within each nanowire.
6. The method of claim 4, wherein the growth conditions are varied
over time by controlling one or more of parameters associated with:
precursor composition, precursor molar flow, carrier gas flow,
temperature, pressure or dopants, or the size distribution of the
catalytic seed particles is varied, such that nanowires with
different properties are formed.
7. The method of claim 1, wherein the catalytic seed particles are
provided as an aerosol that is mixed with the gaseous
precursors.
8. The method of claim 1, wherein the catalytic seed particles are
provided by formation from gaseous reactants that comprises at
least one of the constituents of the catalytic particles.
9. The method claim 1, wherein the gas containing the catalytic
seed particles flows sequentially through one or more reaction
zones, each reaction zone contributes to the nanowire growth by
adding material to the nanowire, and the nanowires grown after
passage through each reaction zone are carried by the gas.
10. The method of claim 1, wherein the catalytic seed particles are
charged.
11. The method of claim 1, further comprising in-situ analysis of
the nanowires formed.
12. The method of claim 11, further comprising controlling the
nanowire growth by feedback from in-situ analysis parameters
without interrupting the nanowire forming process.
13. The method of claim 11, wherein the in-situ analysis comprises
illumination of the nanowires formed and detection of luminescence
from the nanowires to determine optical properties of the
nanowires.
14. The method of claim 1, further comprising depositing and/or
aligning the nanowires on a substrate.
15. The method of claim 1, wherein the nanowires comprise a first
portion and a second portion, wherein the first portion has a first
composition or a first conductivity type and the second portion has
a second composition or a second conductivity type, wherein the
first composition or the first conductivity type is different from
the second composition or the second conductivity type.
16. The method of claim 1, wherein: the metal catalytic seed
particles comprise gold catalytic seed particles; the Group III and
Group V gaseous precursors comprise gallium and arsenic containing
precursors; and dissolving the Group III material into the metal
catalytic seed particles at a temperature of 380.degree. C. to
700.degree. C. comprises dissolving gallium into gold catalytic
seed particles to form Au--Ga seed particles.
17. The method of claim 16, wherein the metal catalytic seed
particles comprise molten catalytic seed particles.
18. The method of claim 16, wherein a gaseous precursor is mixed
with the gold catalytic seed particles prior to initiation of
nanowire growth.
19. The method of claim 1, wherein the at least one nanowire
crystal comprises a gallium arsenide nanowire crystal.
20. The method of claim 1, wherein the nanowires have a formula
GaAs or In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y.
21. The method of claim 1, wherein the nanowires comprise a pn or
pin junction in which a p or n doped nanowire segment is grown
axially on another one of the p or n doped segment.
22. The method of claim 1, wherein a ratio of the Group V precursor
to the Group III precursor is between 0.2 and 5.
23. The method of claim 1, wherein growing epitaxially at least one
nanowire crystal occurs at a reactor pressure of between 50 and
1100 mbar.
Description
CROSS-REFERENCE TO OTHER APPLICATIONS
This application is a national phase application under 35 U.S.C.
.sctn.371 of international application PCT/SE2011/050599 (filed May
11, 2011) which claims priority to Swedish Application 1050466-0
(filed May 11, 2010).
TECHNICAL FIELD OF THE INVENTION
The present invention relates to formation of wires and in
particular to gas-phase synthesis of wires in the absence of a
substrate. The gas-phase synthesis is applicable to different
materials, and in particular to semiconductor materials.
BACKGROUND OF THE INVENTION
Small elongated objects, usually referred to as nanowires,
nanorods, nanowhiskers, etc. and typically comprising semiconductor
materials, have up till now been synthesized using one of the
following routes: liquid phase synthesis, for example by means of
colloidal chemistry as exemplified US 2005/0054004 by Alivisatos et
al, epitaxial growth from substrates, with or without catalytic
particles as exemplified by the work of Samuelson et al presented
in WO 2004/004927 A2 and WO 2007/10781 A1, respectively, or gas
phase synthesis by means of a laser assisted catalytic growth
process as exemplified by WO 2004/038767 A2 by Lieber et al.
The properties of wires obtained using these routes are compared in
the following table.
TABLE-US-00001 Width/ length Scalability/ Material and size
Structural cost of quality control complexity production Liquid
HIGH THIN/ LOW HIGH/HIGH phase SHORT MEDIUM control Substrate- HIGH
ALL/ALL HIGH LOW/HIGH based HIGH control Laser MEDIUM THIN/ LOW
MEDIUM/ assisted LONG MEDIUM/ MEDIUM control
Consequently, the choice of synthesis route is a compromise between
different wire properties and cost of production. For example
substrate-based synthesis provides advantageous wire properties but
since wires are formed in batches the scalability of the process,
and thus the production cost and through-put, are limited.
SUMMARY OF THE INVENTION
In view of the foregoing one object of the invention is to provide
a method and a system for forming wires that enables a large scale
process combined with a structural complexity and material quality
comparable to wires formed using substrate-based synthesis.
The method comprises the basic steps of
providing catalytic seed particles suspended in a gas,
providing gaseous precursors that comprises constituents of the
wires to be formed,
passing the gas-particle-precursor mixture through a reactor,
typically a tube furnace, and
growing the wires from the catalytic seed particles in a gas-phase
synthesis including the gaseous precursors while the catalytic seed
particles are suspended in the gas.
In a first aspect of the invention wires of different configuration
such as wires made of essentially the same material, unipolar
wires, or more complex wires such as wires with axial pn- or
pin-junctions, wires with radial pn- or pin-junctions,
heterostructure wires, etc. can be provided by varying the growth
conditions during growth of each wire, such that a wire segment is
axially grown on a previously formed wire portion in a longitudinal
direction thereof, or a shell is radially grown on the previously
formed wire portion in a radial direction thereof, or material is
added as a combination of axial and radial growth. The growth
conditions can be varied between the reaction zones by controlling
one or more of parameters associated with: precursor composition,
precursor molar flow, carrier gas flow, temperature, pressure or
dopants. This variation is in practice achieved by performing the
wire growth in two or more zones, which may be kept at different
temperature, and into which suitable growth or dopant precursor
molecules are injected by means of mass flow controllers or similar
devices.
Growth conditions can also be varied over time by controlling one
or more of parameters associated with: precursor composition,
precursor molar flow, carrier gas flow, temperature, pressure or
dopants, or the size distribution of the catalytic seed particles,
such that the wire properties can be varied from time to time,
either to produce a batch with a range of different wires, or to
produce distinct homogeneous batches.
The catalytic seed particles can be provided as an aerosol that is
mixed with the gaseous precursors prior to, or during, initiation
of wire growth. Alternatively the catalytic seed particles are
formed by formation from gaseous reactants that comprises at least
one of the constituents of the catalytic particles, thereby
enabling a self-catalyzed wire growth.
Preferably, the method of the invention comprises providing a flow
of the gas that carries the catalytic seed particles and
subsequently the partly or fully formed wires through one or more
reactors, each reactor comprising one or more reaction zones.
Thereby the catalytic seed particles and any wires formed thereon
flow sequentially through one or more reaction zones, where each
reaction zone contributes to the wire growth by adding material to
the wire or etching the wire. This enables to provide optimum
conditions for each step in the growth process.
The diameter of the wires is partly determined by the size of the
catalytic particles. Thus the diameter of the wires can be
controlled by choosing an appropriate size or size distribution of
the catalytic seed particles and by adjusting the growth conditions
to the size of the catalytic seed particles.
In the case of a second reaction furnace or reaction zone,
continued wire growth occurs on pre-fabricated semiconductor wires
with attached catalytic particles, formed in the first reactor.
These wires act as flying substrates, and consequently growth will
take place more readily than in the first zone, where wire
nucleation takes place on the seed particles. Therefore, wire
growth in subsequent furnaces is more efficient and takes place at
lower temperatures. Depending on growth conditions (reactor
temperature and pressure, precursor type and concentration, seed
particle/wire size and concentration, and reaction time) the
subsequent wire growth takes place in the axial or radial
direction, or as a combination of both.
In one aspect of the invention, the method comprises addition of
HCl or other etching halide compound to the flow of aerosol, to
emulate the conditions in hydride vapour phase epitaxy, HVPE,
preventing growth on the hot wall of the reactor. HVPE sources,
where metallic group-III atoms are carried as chlorides to the
reaction zone, can also be used in this invention.
In another aspect of the invention, the seed particles/wires are
heated by means of microwaves, infrared light or other
electromagnetic radiation, instead of or as a complement to the hot
wall tube furnace. This allows the gas to remain more or less cold,
minimizing the amount of gas-phase reactions, while allowing growth
on the hot particle/wire surfaces.
In yet another aspect of the invention the method comprises in-situ
analysis of the wires or the partly grown wires to obtain the
desired wire properties. Means for controlling the wire growth
involve control of the size of the catalytic seed particles, but
also control of growth conditions by controlling one or more of
parameters associated with: precursor composition, precursor molar
flow, carrier gas flow, temperature, pressure or dopants, in one or
more of the reaction zones mentioned above. The in-situ analysis
provides means for obtaining feed-back in a control loop not
available in for example substrate-based synthesis. Any deviation
from desired properties is rapidly detected and the growth
conditions can be adjusted without significant delay or without
having to discard a significant number of wires.
Means for in-situ analysis include means for detecting the size of
the catalytic seed particles and/or the wires formed, such as a
differential mobility analyser (DMA), illumination and detection of
luminescence from the wires formed, absorption spectroscopy, Raman
spectroscopy and X-ray powder diffraction on-the-fly, etc. In
addition to the possibility to control the wire growth in
"real-time" the in-situ analysis can also be used to selectively
sort wires having different properties, such as size. Although
described in terms of wires, it should be appreciated that the
in-situ analysis can be performed also on catalytic seed particles,
or partly formed wires.
In yet another aspect of the invention the method comprises
collection of the wires from the gas that carries the wires. The
wires can be collected and stored for later use or they can be
transferred to a different carrier or a substrate to be
incorporated in some structure to form a device.
To take advantage of the continuous flow of wires the wires may be
deposited and/or aligned on a substrate in a continuous process,
such as a roll-to-roll process. The deposition and/or alignment can
be assisted by an electric field applied over the substrate and
further by charging the wires, and optionally also the substrate.
By local charging of the substrate in a predetermined pattern wires
can be deposited in predetermined positions on the substrate. Thus
the present invention provides a continuous, high through-put,
process for manufacturing aligned wires on a substrate, optionally
with "real-time" feed-back control to obtain high quality
wires.
The wires produced by the method of the invention can be utilised
to realise wire based semiconductor devices such as solar cells,
field effect transistors, light emitting diodes, thermoelectric
elements, field emission devices, nano-electrodes for life
sciences, etc which in many cases outperform conventional devices
based on planar technology.
Although not limited to nanowires, semiconductor nanowires produced
by the method of the invention possess some advantages with respect
to conventional planar processing. While there are certain
limitations in semiconductor devices fabricated using planar
technology, such as lattice mismatch between successive layers,
nanowire formation in accordance with the invention provides
greater flexibility in selection of semiconductor materials in
successive segments or shells and hence greater possibility to
tailor the band structure of the nanowire. Nanowires potentially
also have a lower defect density than planar layers and by
replacing at least portions of planar layers in semiconductor
devices with nanowires, limitations with regards to defects can be
diminished. Further, nanowires provide surfaces with low defect
densities as templates for further epitaxial growth. As compared to
substrate-based synthesis lattice mismatch between substrate and
wire does not have to be considered.
The apparatus of the invention comprises at least one reactor for
growing wires, said reactor comprising one or more reaction zones,
means for providing catalytic seed particles suspended in a gas to
the reactor, means for providing gaseous precursors that comprises
constituents of the wires to be formed to the reactor, and means
for collecting wires grown from the catalytic seed particles in a
gas-phase synthesis including the gaseous precursors while the
catalytic seed particles are suspended in the gas.
A plurality of reactors, each providing a reaction zone, or
reactors that are divided into different reaction zones, or a
combination thereof can be used to enable change of growth
conditions during growth of each wire. During processing the
catalytic particles, the partly grown wires and the fully grown
wires are carried by a gas flow sequentially through the
reactors.
Preferably the apparatus further comprises means for in-situ
analysis of the wires formed. In one embodiment of the invention
said means for in-situ analysis is arranged for detection of wire
properties after one of said reaction zones and a signal from said
means for in-situ analysis is fed back to a means for controlling
the growth conditions upstream.
One advantage of the method and apparatus in accordance with the
invention is that wires can be grown at a surprisingly high rate.
Growth rates may be higher than 1 .mu.m/s, which implies a growth
time of a few seconds for a typical wire of 0.4.times.3 .mu.m
dimension. This means that, in a continuous process in accordance
with the invention the through-put is tremendous.
Embodiments of the invention are defined in the dependent claims.
Other objects, advantages and novel features of the invention will
become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying
drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described with
reference to the accompanying drawings, wherein
FIG. 1 schematically illustrates axial growth of a nanowire in
accordance with the invention,
FIG. 2 schematically illustrates a system for forming wires, in (a)
with a single reactor, and in (b) extended to a modular system with
a plurality of reactors, in (c-h) examples of different sub-modules
in accordance with the invention,
FIG. 3 schematically illustrates axial growth of a wire comprising
a pn-junction in accordance with the invention,
FIG. 4 schematically illustrates core-shell growth of a wire
comprising a pn-junction in accordance with the invention,
FIG. 5 schematically illustrates a system for forming wires
comprising in-situ analysis modules in accordance with the
invention,
FIG. 6 schematically illustrates a first embodiment of a system for
forming nitride based LED structures with different emission
wavelengths in accordance with the invention,
FIG. 7 schematically illustrates a second embodiment of a system
for forming nitride based LED structures with different emission
wavelengths in accordance with the invention,
FIG. 8 schematically illustrates an arrangement for in-situ photo
luminescence measurements in a system for forming wires in
accordance with the invention,
FIG. 9 schematically illustrates an arrangement for in-situ
absorption measurements in a system for forming wires in accordance
with the invention, and
FIG. 10 and FIG. 11 shows wires of different configuration formed
in a system in accordance with the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
For the purpose of this application the term wire refers to an
elongated object. As mentioned above, these wires may be of
essentially nanometer dimensions in their width or diameter,
commonly referred to as nanowires, nanowhiskers, nanorods, etc.,
however not limited to this.
Referring to FIG. 1, basically a method for forming wires in
accordance with the invention comprises providing catalytic seed
particles 2 suspended in a gas, providing gaseous precursors 3, 4
that comprises constituents of the wires 1 to be formed, and
growing the wires 1 from the catalytic seed particles 2 in a
gas-phase synthesis including the gaseous precursors 3, 4 while the
catalytic seed particles are suspended in the gas.
The growth, or at least part thereof, is performed at an elevated
temperature, typically in a furnace or some other kind of reactor,
and starts with an initiation of the growth by catalytic
decomposition of the gaseous precursors 3, 4 on the surface of the
catalytic seed particles 2 and nucleation. After nucleation the
wire 1 grows directionally and forms an elongated object, i.e. a
wire. Preferably the gas flows through the reactor and thereby
carries at least the catalytic seed particles and thus the wires
formed on the catalytic seed particles through the reactor.
The method is described herein in terms of semiconductor materials,
in particular III/V-materials, however not limited to this. By way
of example, FIG. 1 schematically illustrates the formation of a
GaAs wire 1 from a catalytic seed particle 2, such as gold, and
gaseous precursors TMGa 3 and AsH.sub.3 4. As shown the catalytic
particles are carried forward by the gas into a reactor where the
gaseous precursors 3, 4 are present and the reaction takes place.
The precursor gases may be added to the gas flow prior to entering
the reactor or directly to the reactor.
A basic system for forming wires in accordance with the present
invention is schematically illustrated in FIG. 2a. The system
comprises at least one reactor 8 for growing wires 1, means 9 for
providing catalytic seed particles suspended in a gas to the
reactor 8, and means 10 for providing gaseous precursors 3, 4 that
comprises constituents of the wires 1 to be grown from the
catalytic seed particles in a gas-phase synthesis including the
gaseous precursors while the catalytic seed particles are suspended
in the gas. Optionally the system further comprises means 11 for
collecting the wires 1. The system may also comprise means for
in-situ analysis 12 of particles and wires formed in the reactor 8,
such as differential mobility analyzers (DMAs) or other analysis
tools to monitor the dimension or other properties of the
wires.
In one embodiment of the method of the invention the wire growth is
performed in one or more reactors arranged in sequence and/or in
parallel, where a continuous flow of catalytic seed particles is
supplied as an aerosol, which is mixed with gaseous precursors 3,
4, and then the gas mixture enters a first reactor of said one or
more reactors where the wire growth is initiated. The catalytic
seed particles 2 can also be formed by gaseous reactants inside
said first reactor, thereby enabling a self-catalyzed wire growth.
When performing the wire growth in a plurality of reactors, each
reactor increases the complexity of the wires, e.g., to make
pn-junctions or heterostructures in the axial or radial
direction.
The reactors, the means for providing catalytic seed particles,
means for in-situ analysis, etc. of said system do not have to be
separate chambers or arrangements. Preferably the system is a
modular system combined in an in-line production apparatus. In
particular, each reactor may comprise one or more reaction zones
arranged in sequence and/or in parallel as described for the
reactors above. Hence since a reaction zone has the same function
as a reactor, these terms are interchangeably used hereinafter.
FIG. 2b schematically illustrates such a modular system with
particle delivery system 9, several growth modules arranged in
series and in parallel and means for collecting the particles and
wires being carried out from the growth modules by the gas flow.
FIG. 2 shows other examples of modules that can be incorporated in
the system: (c) a wire growth module, (d) a shell growth module,
(e) a passivation layer growth module, (f) an in-situ analysis tool
12 (with the arrow indicating the possibility to feed-back
control), such as a DMA, (g) an evaporation module with an
evaporation source 13 and (h) a plasma-enhanced chemical vapour
deposition module with a plasma source 14, however not limited to
this.
FIG. 3 schematically illustrates how the method of the invention
can be used to form a GaAs wire comprising an axial pn-junction
between a p-doped GaAs segment and an n-doped GaAs segment.
Precursors 3, 4 comprising group III material and group V material,
respectively, and p-dopants are provided to a reactor and, after
nucleation, p-doped GaAs is axially grown from the catalytic seed
particle, thereby forming a first axial segment of the GaAs wire.
Thereafter the growth conditions are changed by exchanging the
p-dopant to an n-dopant, while substantially maintaining other
parameters related to the growth conditions, such that a second
axial wire segment is axially grown on the previously formed first
segment in a longitudinal direction thereof. This illustrates the
possibility to vary the growth conditions during axial growth to
obtain axial segments with different properties.
FIG. 4 schematically illustrates the formation of a GaAs wire
comprising a radial pn-junction between a p-doped GaAs core and an
n-doped GaAs shell. Precursors 3, 4 comprising group III material
and group V material, respectively, and p-dopants are provided to
the reactor and after nucleation p-doped GaAs is axially grown from
the catalytic seed particle, thereby forming the core of the GaAs
wire. Thereafter the growth conditions are changed by increasing
the temperature and/or the V/III-ratio to promote radial growth and
by exchanging the p-dopant to an n-dopant. Thereby the shell is
radially grown on the previously formed core in a radial direction
thereof. This illustrates the possibility to vary the growth
conditions to switch between axial growth and radial growth.
Although exemplified with GaAs, it should be appreciated that other
III/V semiconductor materials as well as semiconductor materials
comprising group II and group VI materials can be processed in the
same way. For example the gaseous precursors of the above examples
can be exchanged for TMIn and PH.sub.3 to form InP wires. As
appreciated to a person skilled in the art the reactor
configuration does not have to be changed to form wires from
different gaseous precursors, the gaseous precursors are simply
switched. Moreover, the processes such as those exemplified by FIG.
3 and FIG. 4 can be performed with or without dopant. Insulators
can also be grown. Single or multiple reactors or reaction zones
within a reactor can be used to improve formation of segments,
cores or shells having different composition, doping or
conductivity type. Moreover, axial and radial growth is not
necessarily fully decoupled but the wire can grow both radially and
axially at the same time. By choosing appropriate gaseous
precursor, flows, temperatures, pressures, and particle sizes, the
wire material can be made to grow in the axial or radial direction,
or in a combination of the two growth modes.
The catalytic seed particles may consist of a single element, or a
combination of two or more elements, to assist in the wire growth
or dope the wire. Gaseous precursors may also be used to dope the
wire.
In case of pre-forming the catalytic seed particles said means for
providing catalytic seed particles 9 may comprise a particle
generator. The particle generator produces an aerosol of more or
less size-selected particles by a range of prior art methods.
Particle generation can be done by evaporation/condensation, spray
or vapor pyrolysis, spark discharge, laser ablation,
electrospraying of colloidal particles, etc. Size selection can be
done by gas mobility classification, e.g. by using a DMA, virtual
impaction, or simply well-controlled particle formation. For many
applications, it is desirable that the aerosol particles be
electrically charged, which can be accomplished by radioactive
sources, corona discharge, thermal or optical emission of
electrons, etc. A typical system for particle generation is
described in Magnusson et al., Gold nanoparticles: production,
reshaping, and thermal charging, J Nanoparticle Res 1, 243-251
(1999).
As mentioned above, the system may comprise one or more reactors or
reaction zones, where each reactor or reaction zone adds a new
functional layer to the wires. Such a modular system is shown in
FIG. 5, and is further described below. Depending on the growth
parameters, such as precursor molecules, temperature, pressure,
flows, particle density and particle size, the new functional layer
can be added as an axial extension of the previously formed wires,
as a radial shell, or as a combination of both axial and radial
growth. The formed layers can be of similar or dissimilar material,
i.e., homo- or heteroepitaxy, and of similar or dissimilar
conduction type, e.g., a pn-junction. The functional layers are not
limited to crystalline layers formed by epitaxy but can also be
amorphous layers such as oxides, providing a passivating and/or
insulating functionality. Chemical reactions to coat the wires with
surfactants or a polymer shell, or condensation of sacrificial
layers for later re-dispersion are other possibilities.
For some growth conditions, additional modules may be added to the
reactor or the reaction zone. For example a plasma generator may be
added to modify the chemical reactions to enable higher reaction
rates. This is important especially if the wire or layer formed on
the wire is grown at low temperature by a stable precursor which
usually requires a high temperature to decompose. A typical example
where this may be useful is for growth of nitrides from
ammonia.
Before or between the reactors or reaction zones, further
components may be placed, for example means for charging particles
or wires. A tube-shaped absorption filter can be used to remove
precursor molecules and small particles from the gas flow, by
taking advantage of a comparatively low diffusion coefficient of
the wires. Precursors and reactants can thereby be replaced, not
only added, between the growth reactors. Size classification tools,
such as DMA or virtual impactor, can also be used to refine the gas
flow, i.e. the aerosol, or as in-situ analysis as explained
below.
Referring to FIG. 5, in the following one implementation of the
method of the invention is described in terms of growing GaAs wires
containing a pn-junction. The system comprises a particle delivery
system that can consist of any of the prior mentioned particle
generators. The particles are generated and then carried by a gas
flow such as H.sub.2 or N.sub.2 from the particle delivery system.
Hereinafter, the gas flow containing particles or wires is termed
an aerosol. The GaAs (n-type) wire growth module consists of a
reaction furnace and a gas delivery system for the precursor
molecules. In this case the precursor molecules are TMGa, AsH.sub.3
and SiH.sub.4. TMGa and AsH.sub.3 form the GaAs material while
SiH.sub.4 dopes the wires with Si resulting in an n-type material.
The precursor molecules are mixed with the aerosol prior to
entering the reaction furnace. Upon entering the reaction furnace
the precursors react with the particles in the aerosol forming
n-type GaAs wires. The growth parameters (temperature, flows,
pressure etc.) are modified to obtain the desired properties
(length, crystal structure, shape etc.). After the GaAs (n-type)
wire growth module, the aerosol, which now consists of the carrier
gas and n-type GaAs wires, exits the GaAs wire growth module, and
is divided into a small flow and a large flow. The small flow
enters a DMA which analyzes the wires size distribution. The larger
flow enters the next wire growth module. The GaAs (p-type) wire
growth module is designed to grow an axial extension of p-type GaAs
on top of the previously grown n-type GaAs wires. The growth module
has essentially the same design as the GaAs (n-type) wire growth
module except for the precursors which now consist of TMGa,
AsH.sub.3 and DEZn. TMGa and AsH.sub.3 form the axial extension of
GaAs material while DEZn dopes the wires with Zn resulting in a
p-type material. The growth parameters in this furnace are not
necessarily the same as in the previous growth module but are
instead optimized to obtain an axial extension of the wires with a
high quality p-type GaAs material. Upon exiting the GaAs (p-type)
wire growth module the aerosol is divided in a small and a large
flow. The small flow enters a DMA which analyzes the wires size
distribution. The large flow enters a wire collection module which
can collect the wires by any of the prior mentioned methods.
By using a plurality means for in-situ analysis, such as the two
in-situ DMAs of FIG. 5, the wire growth process can be monitored at
intermediate states of the wire growth and if necessary, growth
parameters can be adjusted to obtain consistent, high quality wires
with the desired properties.
As mentioned above, said method and system of the invention can be
used to form complex wire structures. By way of example, FIG. 6
schematically illustrates a system for growth of nitride-based
light emitting diodes (LEDs) adapted to provide emission at
different wavelengths. The system comprises a particle delivery
system a GaN (n-type) wire growth module arranged in series
followed by InGaN shell growth modules arranged in parallel prior
to an AlGaN (p-type) shell module and finally a means for
particle/wire collection. Hence, the gas flow is divided into
parallel InGaN shell growth modules that are adapted to form InGaN
shells having different composition, i.e. In.sub.xGa.sub.1-xN,
In.sub.yGa.sub.1-yN and In.sub.yGa.sub.1-yN where
x.noteq.y.noteq.z. Due to the different growth conditions in each
of the branches the wires will obtain different emission
characteristics. For example, wires adapted for emission in the
red, green and blue wavelength regions can be accomplished. By
collecting the at least partly formed wires from the InGaN shell
growth modules into a common gas flow the different wires can be
grown and collected simultaneously for assembly of white light
LEDs.
FIG. 7 schematically illustrates a similar system as shown in FIG.
6, although with the possibility of more control during growth
since different InGaN quantum wells get different shells
individually adapted for the quantum well structure. In addition to
the parallel InGaN shell growth modules of the system of FIG. 6,
each InGaN shell growth module is followed by a p-AlGaN growth
module. However, a n-GaN wire growth module and a AlO passivation
layer growth module following the p-AlGaN shell growth modules may
be the same for different wires in order to reduce the complexity
of the system.
The flexibility of the system allows for several in-situ analysis
tools 12, to measure and monitor properties which are not
obtainable using other wire growth techniques. This allows instant
feedback to regulate the system, making it possible to continuously
fine-tune material parameters in a way that is not possible in
other methods.
By way of example, wire size measurement and sorting is achievable
by using a DMA. The DMA, or any other means for in-situ analysis,
can be coupled either in series or in parallel, depending on if the
measurement is to be invasive or non-invasive on the gas flow.
Coupled in series a DMA can sort the wires in the aerosol by their
size. The size and size distribution which is sorted depends on the
properties and settings of the DMA. Coupled in parallel, a small
aerosol flow can be extracted to the DMA for an almost non-invasive
measurement. In this case the DMA can scan within its size
detection range to give the size distribution of the aerosol. This
can be done while only wasting a small part of the gas flow thus
maintaining a high production rate of wires.
By illuminating the gas flow, the optical properties of the wires
can be studied in a non-invasive manner. The light source should
preferably be a laser where the energy of the light is higher than
the band gap of one or more materials that the wires consist of. By
using a photodetector, the luminescence from the wires can be
studied. This enables monitoring of the optical properties of the
wires, which can be used to tune growth parameters to obtain the
desired properties of the wires. This is in contrast to other
growth methods in that the wires may be cooled down rapidly after
each successive growth reactor or reaction zone and the temperature
sensitive photoluminescence technique can be used between each step
in the wire growth.
Further possible in-situ optical methods include absorption
spectroscopy, where the absorption path would ideally be along the
wire flow; Raman spectroscopy (especially Coherent anti-Stokes
Raman Spectroscopy, CARS), which can also be used inside reaction
furnaces to study decomposition of molecules and temperature
gradients; and X-ray powder diffraction on-the-fly.
Depending on the type of wires being produced, different collection
methods are possible. For charged wires, they are easily collected
on any substrate by means of an electric field. The aerosol may be
bubbled through a liquid to remove the wires from the gas flow,
with or without surfactant molecules to keep the wires from
agglomerating. Wires that are easily re-dispersed may be collected
in a filter as a dry powder.
FIG. 8 schematically illustrates an arrangement for in-situ photo
luminescence (PL) measurements in a system for forming wires in
accordance with the invention. This PL arrangement comprises a
light source and a photodetector arranged at e.g. a transparent
quartz tube. For an appropriate luminescence measurement the light
source should be a laser with light of higher energy than the
bandgap of the semiconductor material of the wires flowing through
said transparent quartz tube.
FIG. 9 schematically illustrates an arrangement for in-situ
absorption measurements in a system for forming wires in accordance
with the invention. This in-situ absorption measurement arrangement
comprises a light source and an absorption detector arranged at
e.g. a transparent quartz tube. For an absorption measurement the
light should emanate from a white light source with collimated
light. The absorption detector is preferentially placed in this
alignment to the light source in order to maximize the absorption
volume of the aerosol.
As a further example of wires formed by the method and the system
of the invention FIG. 10 and FIG. 11 shows scanning electron
microscope (SEM) images of GaAs nanowires grown under two different
growth conditions, hereinafter referred to as (i) and (ii),
respectively. Au agglomerates are generated from molten Au in a
high temperature furnace with a set temperature of (i) 1775.degree.
C. or (ii) 1825.degree. C. The Au agglomerates are carried by 1680
sccm of N.sub.2 carrier gas (hereinafter the carrier gas containing
Au agglomerates/particles is termed aerosol) between the different
modules of the growth system. After the high temperature furnace
the Au agglomerates are charged with a single electron each. By
using this single electron charge the Au agglomerates are size
selected by a differential mobility analyzer, in this case set at
50 nm. The aerosol is passed through a sinter furnace with a
temperature of 450.degree. C. which compacts the Au agglomerates
into spherical Au particles. After the sinter furnace the aerosol
is mixed with the precursor gases TMGa and AsH.sub.3, with a set
molar flow of 2.4*10.sup.-2 mmol/min and 2.2*10.sup.-2 mmol/min
respectively. The aerosol, including the precursor gases, enters
the reaction furnace set to a temperature of (i) 450.degree. C. or
(ii) 625.degree. C. Inside the reaction furnace the precursors
decompose to form the material constituents Ga and As. The material
constituents are supplied to the Au particles in the gas phase and
a GaAs seed crystal is nucleated on the Au particle. The continued
growth of the wire proceeds via two different growth modes, (i) an
axial growth mode where material is incorporated in the interface
between the Au particle and the GaAs seed crystal forming a wire,
(ii) a combination of an axial and radial growth mode where
material constituents are incorporated both at the Au particle-GaAs
interface and on the side facets of the wire that is formed,
forming a wire with a conical shape. After the reaction furnace the
wires are transported by the carrier gas to a deposition chamber
where a voltage of 6 kv is applied to a Si substrate to deposit the
electrically charged wires. As shown in FIG. 10 the Au particle is
visible and has a bright contrast compared to the darker nanowires.
As shown in FIG. 11 the Au particle is visible having a bright
contrast at the tip of the conically shaped nanowire.
The formation of GaAs nanowires typically takes place in the
temperature regime between 380.degree. C. and 700.degree. C.
depending on the desired shape and properties of the formed
nanowires. A higher temperature typically results in a higher
growth rate, i.e., longer nanowires for a set growth time, but also
in a conical shape, along with effects on crystal structure and
impurity incorporation. Besides temperature, the ratio of group V
material precursor to group III material precursor, i.e., the V/III
ratio, is important. If the V/III ratio is too low, typically below
0.2, the nanowire growth proceeds in a group III rich environment
which can reduce the growth rate and material quality. If the V/III
ratio is too high, typically above 5, the nanowires are difficult
to nucleate since group III material can't be dissolved in the Au
particles. Formation of GaAs nanowires typically takes place with a
total pressure inside the reactor between 50 and 1100 mbar. A lower
pressure reduces the supersaturation in the gas phase which can
reduce parasitic gas phase reactions. A higher pressure increases
the supersaturation in the gas phase which can increase the
supersaturation in the Au particle and increase the growth rate.
The pressure can also be used to control the residence time in the
growth reactor.
It should be noted that parameters such as temperature, precursor
flow, V/III ratio and pressure are dependent on the precursor
molecules that are used since only the material that actually
reaches the growth interface is incorporated. If a precursor can
withstand higher temperatures without reacting, the
nanowire-forming reaction most likely takes place at a higher
temperature.
The above discussion on growth parameters is valid mainly for
single stage growth, where nucleation and wire growth take place in
a single reaction zone. For multiple stage growth, the first
nucleation stage should typically be done at a higher temperature,
lower precursor flow and lower V/III ratio, compared to the
subsequent growth steps.
Compared with MOVPE nanowire formation in the described process
typically takes place at a lower V/III ratio but at similar
temperatures. Since parameters such as temperature, pressure, flows
and V/III ratio are dependent on the exact chemistry used to form
the nanowires it is understood that different materials may be
formed at different parameters. For example III-nitrides may be
formed at higher temperatures due to the higher stability of the
NH.sub.3 precursor, while InAs growth is done at lower
temperatures.
Suitable materials for formation of the wires of the method and the
system in accordance with the invention include, but are not
limited to: InAs, InP, GaAs, GaP and alloys thereof
(In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y) InSb, GaSb and alloys thereof
(In.sub.xGa.sub.1-xSb) AlP, AlAs, AlSb and alloys thereof for
example AlP.sub.1-xAs.sub.x. InGaAsP alloyed with Al, for example
Al.sub.xGa.sub.1-xAs InGaAsP alloyed with Sb, for example
GaAs.sub.ySb.sub.1-y InN, GaN, AlN and alloys thereof
(In.sub.xGa.sub.1-xN) Si, Ge and alloys thereof, i.e.
(Si.sub.xGe.sub.1-x) CdSe, CdS, CdTe, ZnO, ZnS, ZnSe, ZnTe, MgSe,
MgTe and alloys thereof SiO.sub.x, C (Diamond), C (Carbon nanotube)
SiC, BN
Suitable materials for the catalytic seed particle include, but are
not limited to: Au, Cu, Ag In, Ga, Al Fe, Ni, Pd, Pt Sn, Si, Ge,
Zn, Cd Alloys of the above, e.g., Au--In, Au--Ga, Au--Si
Suitable gases for carrying the catalytic seed particles and the
wires in the process include, but are not limited to: H.sub.2,
N.sub.2 or a mixture thereof; or He, Ar.
Suitable dopants include, but are mot limited to, for InGaAl--AsPSb
system: n-dopants: S, Se, Si, C, Sn; p-dopants: Zn, Si, C, Be
AlInGaN system: n-dopants: Si; p-dopants: Mg Si: n-dopants: P, As,
Sb; p-dopants: B, Al, Ga, In CdZn--OSSeTe system: p-dopants: Li,
Na, K, N, P, As; n-dopants: Al, Ga, In, Cl, I
According to common nomenclature regarding chemical formula, a
compound consisting of an element A and an element B is commonly
denoted AB, which should be interpreted as A.sub.xB.sub.1-x
It should be appreciated that the wire growth may comprise one or
more etch steps, where material is removed rather than grown on the
wires. Etching can also be used to decouple radial and axial
growth, which for example enables lowering of the tapering of the
wires or simple shape control of the wires.
The size of the wires depends on many factors such as the materials
forming the wires, the intended application for the wires and the
requirement on quality of the wires formed. Preferably the wires
have diameter of less than 10 .mu.m, and more preferably, in
particular for formation of wires comprising lattice mismatched
layers or segments, the wire diameter is less than 300 nm.
Since the wires of the invention may have various cross-sectional
shapes the diameter, which interchangeably is referred to as width,
is intended to refer to the effective diameter.
While the invention has been described in connection with what is
presently considered to be the most practical and preferred
embodiments, it is to be understood that the invention is not to be
limited to the disclosed embodiments, on the contrary, it is
intended to cover various modifications and equivalent arrangements
within the appended claims.
* * * * *